1. Field of the Invention
This invention relates to a process and apparatus for the ram extrusion of plastic pipe, especially the ram extrusion of thermoplastic high molecular weight low pressure polyethylene of molecular weight of more than 500,000. This invention also relates to such pipe or tube of high molecular weight low pressure polyethylene and to the use of such pipe or tube as a liner for steel pipe.
2. Discussion of the Prior Art
High molecular weight low pressure polyethylene (UHMW-PE) having a molecular weight of more than 500,000 (determined by R.S.V Iso/R 1191) differs fundamentally from usual polyethylenes by its melting and viscosity behavior. While transfer into a melt having satisfactory flow properties is achieved in case of polyethylenes having a molecular weight of 200,000 by an increase in temperature, UHMW-PE on an increase in temperature merely turn over to a viscous-elastic state.
The high viscosity and the poor flow properties do not permit processing of this material to pipe on single screw or twin screw extruders with conventional pipe extrusion dies without destruction of the properties of the material. Since the mechanical properties of UHMW-PE (strength, resistance to stress cracking, high impact strength (notched), sliding properties) offer ideal prerequisites for making pipe which is used for pneumatic and hydraulic transport of solids (quartz, sand, coal slurry), it became necessary to provide a process which permits processing of this material to pipe thereby opening to it a new field of use.
It is known that it is possible by means of ram extrusion to manufacture continuously plates or rods from high molecular weight low pressure polyethylene (see, for example, German (BRD) Offenlegungsschrift No. 17 29 174 and German (BRD) Pat. No. 17 78 258. However, pipe of UHMW-PE having the necessary dimensional stability according to Deutsche Industrie Norm (DIN) 8074 and the typical tensile and strength properties of UHMW-PE could not be manufactured heretofore.
The production of polytetrafluoroethylene pipe by means of ram extrusion has been described by Steininger and Stemprech in the paper "Extrusionsverarbeitung von Polytetrafluorathylen-Fullstoff-Mischungen" ("Extrusion processing of Polytetrafluoroethylene-filler compositions") in Kunststoffe 1973, pp. 558 et seq. The pressure developed in pipe production is many times that needed for the production of solid sections. Since the pressures developed in ram extrusion of UHMW-PE are considerably higher than the pressures which are developed in ram extrusion of polytetrafluoroethylene, deterioration of the properties of the material would have to be expected when producing pipe by extrusion of UHMW-PE by the process mentioned above. Moreover, the construction and design of the extruder would become economically unattractive due to the high design pressure.
It is an object of the invention to provide pipe of UHMW-PE while avoiding deterioration of the typical material properties of UHMW-PE. The pipe dimensions should correspond to the tolerances in size specified in DIN 8074. For this purpose, it became desirable to provide a process and an apparatus for ram extrusion of plastic pipe wherein high extrusion pressures are avoided. The construction and design of the ram extruder should be as simple and inexpensive as possible.